Attachment E - GEO Tech Report.pdf

Revised Geotechnical Engineering Report

Proposed New Outpatient Building

Durham VA Hospital

508 Fulton Street

Durham, North Carolina

January 30, 2014

Project No. 70135142

Prepared for:

Guidon Design, Inc.

Indianapolis, Indiana

Prepared by:

Terracon Consultants, Inc.

Raleigh, North Carolina

'||'erracon Guidon Design, lnc.

2453 N. Delaware Street Indianapolis, Indiana 46205

Attn: Mr. Scott Noyer

Re: Revised Geotechnical Engineering Report Proposed New Outpatient Building Durham VA Medical Center 508 Fulton Street Durham, North Carolina Terracon Project No. 70135142

Dear Mr. Noyer, Terracon Consultants, Inc. (Terracon) is pleased to submit the revised geotechnical engineering report for the proposed new outpatient building at the VA Medical Center in Durham, North Carolina. Terracon previously prepared a geotecrnical engineering report for the project dated December 9, 2013. Since that time, our understanding of the project has changed. This report presents the findings of the subsurface exploration and provides geotechnical recommendations concerning earthwork and the design of foundations and floor slabs for the proposed structure.

These services were performed in accordance witr our Proposal No. P71135G037 dated August 16, 2013.

We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report, or if we may be of further service, please contact us. \,,\\=-i.-|=:.,,n

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\\5 0°‘3*»Sincerely, é__.;i,,b 4% '9 __ _-QTerracon Consultants, Inc. E E SEAL.

= 5 034749 1-.“

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-rd» *1/1.» . it f1* ” ~~@~=-v .- Thomas R. Bartlett, El . L.~il'_'e ' enton, , ’*»,,£/D |__D?5:-* Staff Professional Principal I Geotechnical Manager """"

Registered, NC 034749

Terracon Consultants, Inc. 2401 Elrentwood Road, Suite 10? Raleigh, North Carolina 2?6U4 P [919] B?'3 2211 F [919] 8?/3 9555 Terracon.com North Carolina Registered F-0869

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TABLE OF CONTENTS

PAGE

EXECUTIVE SUMMARY ............................................................................................................ i

1.0 INTRODUCTION

2.0 PROJECT INFORMATION

2.1 Project Description

2.2 Site Location and Description

3.0 SUBSURFACE CONDITIONS

3.1 Site Geology

3.2 Typical Profile

3.3 Groundwater

4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION

4.1 Geotechnical Considerations

4.2 Earthwork

4.2.1 Excavations

4.2.2 Acceptable Engineered Fill Materials

4.2.3 Engineered Fill Compaction Requirements

4.2.4 Grading and Drainage

4.2.5 Construction Considerations

4.3 Foundation Recommendations

4.3.1 Foundation Excavation Preparation

4.3.2 Foundation Design Parameters

4.3.3 Uplift Resistance

4.4 Lateral Earth Pressures

4.4.1 Basement Wall

4.4.2 Basement Wall in Conjunction with a Soil Nail Wall

4.5 Seismic Considerations

4.6 Floor Slabs

4.7 Heavy Duty Pavements

5.0 GENERAL COMMENTS

APPENDIX A – FIELD EXPLORATION

Exhibit A-1 Site Location Plan

Exhibit A-2 Boring Location Plan

Exhibit A-3 Subsurface Profile

Exhibit A-4 Field Exploration Description

Boring Logs B-1 through B-5

APPENDIX B – LABORATORY TESTING

Exhibit B-1 Laboratory Testing Description

Exhibit B-2 Atterberg Limits Results

APPENDIX C – SUPPORTING DOCUMENTS

Exhibit C-1 General Notes

Exhibit C-2 Unified Soil Classification

Revised Geotechnical Engineering Report Proposed New Outpatient Building ■ VA Medical Center ■ Durham, North Carolina January 30, 2014 ■ Terracon Project No. 70135142

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EXECUTIVE SUMMARY

Terracon Consultants, Inc. (Terracon) is pleased to submit the revised geotechnical engineering report for the proposed new outpatient building at the VA Medical Center located at 508 Fulton

Street in Durham, North Carolina. This revised report presents the findings of our field exploration and laboratory testing services and makes geotechnical engineering recommendations based on our current understanding of project. The following geotechnical considerations were identified:

Partially weathered rock (PWR) was encountered at elevations of 368 to 392 feet in the borings performed at the site. Excavation of PWR will be required in some portions of the proposed building footprint to reach the subgrade elevation for foundations and floor slabs and PWR may be encountered in utility or other excavations. Penetrating PWR will require additional effort such as ripping, jackhammering, or other means to advance excavations.

The site should be suitable for support of the structure on shallow foundations at an allowable bearing pressure of 3,000 pounds per square foot bearing on weathered rock, fill consisting of crushed aggregate base course (NCDOT CABC) extending to weathered rock, or fill consisting of crushed aggregate base course (NCDOT CABC) extending to a depth of 5 feet below the design foundation bearing elevation. The recommended overexcavation and replacement with CABC is intended to reduce the potential for development of premature distress in the structure due to differential settlement caused by varying stiffness of the subgrade materials at foundation bearing elevations.

If additional uplift resistance is required, tiedown anchors could be utilized. Tiedown anchors are generally installed by drilling or coring into rock and embedding a portion of a threaded bar in grout or epoxy. Once the grout sets, the rod can be tensioned by threading the appropriate bearing plates and nuts.

Many tiedown anchor contractors also install soil nails. Soil nailing is a construction technique used to reinforce and strengthen existing ground to facilitate excavation support. The use of soil nails is currently being considered to reduce the design loads against below grade basement walls.

There are two existing structures within the proposed construction area that will require demolition. Additionally, fill materials associated with the previous development were encountered at the site. The fill encountered in the borings appeared relatively firm and free of debris. Even with the recommended site improvement and additional testing, some existing fill may remain in place in pavement areas. Construction on existing fill

Responsive ■ Resourceful ■ Reliable ii represents an inherent risk for the owner that compressible fill or unsuitable material within or buried by the fill will not be discovered. This risk of unforeseen conditions cannot be eliminated without completely removing the existing fill. However, these risks can be reduced by performing additional testing and evaluation during construction.

We understand the below ground level will include a concrete slab-on-grade. The varied depth to partially weathered rock can create stress concentration within the floor slab and lead to premature distress. To reduce the potential for stress concentrations to develop within the floor slab, and to increase the constructability of utilities, the floor slab design should include a crushed stone base course (NCDOT CABC) layer approximately

12 inches thick that extends to an approved subgrade.

Soils of the Triassic Basin are moisture sensitive and will become soft and unstable when wet. Due to the potential for unstable subgrades during wet weather, it is preferable to perform earthwork operations during late summer and early fall when precipitation durations are shorter and drying times are quicker. Performing site preparation and earthwork at other times of the year increases the potential for having to perform remedial work on the subgrade soils.

We understand that heavy duty pavements will be constructed as part of the proposed project. Prior to pavement construction the pavement subgrade should be proofrolled under observation by a representative of the geotechnical engineer to identify unsuitable areas. We expect that some localized areas may require overexcavation and replacement due to soft conditions.

A qualified geotechnical engineer should be retained during the construction phase of the project to observe earthwork and to perform necessary tests and observations during subgrade preparation; to monitor proof-rolling, foundation excavation operations, placement and compaction of controlled compacted fills, backfilling of excavations to the completed subgrade; and to observe prior to placing reinforcing steel in the footing excavations.

This summary should be used in conjunction with the entire report for design purposes. It should be recognized that details were not included or fully developed in this section, and the report must be read in its entirety for a comprehensive understanding of the items contained herein. The section titled GENERAL COMMENTS should be read for an understanding of the report limitations.

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REVISED GEOTECHNICAL ENGINEERING REPORT

PROPOSED NEW OUTPATIENT BUILDING

DURHAM VA MEDICAL CENTER

508 FULTON STREET

DURHAM, NORTH CAROLINA

Terracon Project No. 70135142

January 30, 2014

1.0 INTRODUCTION

Terracon Consultants, Inc. (Terracon) is pleased to submit the revised geotechnical engineering report for the proposed new outpatient building at the VA Medical Center located at 508 Fulton

Street in Durham, North Carolina. Terracon has previously prepared a geotechnical engineering report dated December 9, 2013 for the proposed new outpatient building and associated aspects of the project. Since that time, our understanding of the project has drastically changed. This report presents the findings of our field exploration and laboratory testing services and makes geotechnical engineering recommendations based on our current understanding of project information relative to:

subsurface soil conditions foundation design and construction groundwater conditions seismic considerations site preparation / earthwork floor slab design and construction pavements lateral earth pressures

2.0 PROJECT INFORMATION

2.1 Project Description

Item Description

Structures New 3-story steel-framed structure with a below grade level and a potential future add-on of 2 additional stories.

Maximum loads

Column Compression – less than 500 kips

Wall Compression - less than 4 kips per linear foot (klf) (assumed)

Lateral – unknown

Uplift – less than 85 kips, may require rock anchors for additional resistance

Grading (assumed)

We expect cuts of up to 15 feet to reach the floor elevation of the below ground level. We expect fill depths of up to 7 feet may be placed at locations adjacent to the structure.

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Finished floor elevation (FFE) The below grade level FFE is expected to be at an elevation of 384 feet. The entry level is expected to be at an elevation of 398 feet.

Retaining / basement walls

We anticipate that basement walls could have unbalanced backfill heights of up to 15 feet. We understand that several options are being considered for the basement walls including installation of (1) a traditional basement wall retaining soil backfill, (2) a permanent soil nail wall acting as a form for a cast-in-place concrete wall, or

(3) a permanent soil nail wall with backfill between the nailed face and independent basement wall.

Pavements Heavy duty pavements are anticipated to be incorporated into the project.

If any of the project information outlined above is inconsistent with the proposed construction, or if the design changes, Terracon requests the opportunity to review our recommendations.

2.2 Site Location and Description

Location

The project site is located on a portion of the existing VA Hospital complex located at 508 Fulton Street in Durham, North Carolina.

Refer to Exhibit A-1 and A-2 for further details regarding project location.

Existing improvements

Two existing structures will be demolished to accommodate the new structure. An existing concrete retaining wall to be removed currently maintains grade separations of approximately 5 to 8 feet between the northern boundary of the construction area and the drive lanes at lower elevations to the north.

Existing topography Moderately sloping down to the north and east.

Existing groundcover The areas surrounding the existing structures are landscaped and include grassed areas and several trees.

3.0 SUBSURFACE CONDITIONS

3.1 Site Geology

According to the 1985 Geologic Map of North Carolina, the site is located within the Triassic

Basin. The Triassic Basin is part of the Piedmont Physiographic Province and was created through the accumulation of eroded sediments in a deep basin that was formed through rift faulting. Over time, these sediments were compressed and partially cemented to form soft to

Responsive ■ Resourceful ■ Reliable 3 moderately hard sedimentary rock. The soils that form the subsurface profiles are generally residual materials derived from the in-place weathering of these rocks.

Partially weathered rock (PWR) is an engineering term used to describe residual material that can be drilled with soil boring methods but would exceed SPT N-values of 100 blows/foot if tested. Partially weathered rock encountered in the Triassic basin geologic setting is generally relatively thick and may consist of moderately hard sedimentary rock or may be derived from the in-place chemical and physical weathering of the parent sedimentary bedrock.

3.2 Typical Profile

Fill materials consisting of silty and clayey sand were encountered to depths of 3 to 4 feet in

Boring Nos. B-3 and B-1, respectively. These materials were generally found to be relatively free of debris and loose to medium dense in relative density. Native soil overburden materials encountered in the borings performed at the site generally consisted of low to highly plastic soil with varying proportions of clay and sand. USCS classifications of CL, CH, and SC are appropriate for these materials which were generally found to be medium stiff to hard in consistency and medium dense to very dense in relative density.

Partially weathered rock and apparent bedrock (identified by auger refusal) were encountered at variable depths in the borings. The following table outlines the depth to weathered rock and bedrock with respect to the boring locations:

Boring No.

Depth to Partially

Weathered Rock

(feet)

Partially Weathered

Rock Elevation

(feet)

Termination Depth

(feet) Termination Type

B-1 10.5 380.5 19.9 Auger Refusal

B-2 5.5 388.5 11.7 Auger Refusal

B-3 10 390 31.4 Boring Terminated

B-4 22 368 33 Auger Refusal

B-5 2 392 13 Auger Refusal

For a detailed description of the conditions encountered in the borings, refer to the boring logs in

Appendix A of this report. Stratification boundaries on the boring logs represent the approximate location of changes in soil types; in-situ, the transition between materials may be gradual.

3.3 Groundwater

The boreholes were observed for the presence and level of groundwater while drilling and after the short period they were allowed to remain open. Groundwater was not encountered in the open boreholes.

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In the Triassic Basin setting, groundwater typically occurs as localized zones of water trapped within the seams and fractures of the parent rock, in sand lenses of the soil mantle, and at the soil / weathered rock interface. The groundwater does not occur as a uniform condition below a given depth or elevation. As a result, the occurrence and location of groundwater can vary dramatically throughout the year due to variations in the amount of rainfall, runoff, and evaporation. Although groundwater inflows were not observed in the borings, the possibility of encountering groundwater should be considered when developing the design and construction plans for trenches. Perched and ponded water conditions can potentially develop on the project site when precipitation infiltrating the surface materials becomes trapped over the less permeable soil and weathered rock below.

4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION

4.1 Geotechnical Considerations

Partially weathered rock (PWR) was encountered at elevations of 368 to 392 feet in the borings performed at the site. Excavation of PWR will be required in some portions of the proposed building footprint to reach the subgrade elevation for foundations and floor slabs and PWR may be encountered in utility or other excavations. Penetrating PWR will require additional effort such as ripping, jackhammering, or other means to advance excavations.

The site should be suitable for support of the structure on shallow foundations bearing at an allowable bearing pressure of 3,000 pounds per square foot on weathered rock, fill consisting of crushed aggregate base course (NCDOT CABC) extending to weathered rock, or fill consisting of crushed aggregate base course (NCDOT CABC) extending to a depth of 5 feet below the design foundation bearing elevation. The recommended overexcavation and replacement with

CABC is intended to reduce the potential for development of premature distress in the structure due to differential settlement caused by varying stiffness of the subgrade materials at foundation bearing elevations.

If additional uplift resistance is required, tiedown anchors could be utilized at the required column locations. Tiedown anchors are generally installed by drilling or coring into rock and embedding a portion of a threaded bar in grout or epoxy. Once the grout sets, the rod can be tensioned by threading the appropriate bearing plates and nuts.

Many tiedown anchor contractors also install soil nails. Soil nailing is a construction technique used to reinforce and strengthen existing ground by installation of horizontal reinforcing elements as a means of supporting excavations. Soil nail walls can be designed as temporary or permanent applications. The use of soil nails is currently being considered to reduce the design loads against below grade basement walls.

Responsive ■ Resourceful ■ Reliable 5 demolition. Additionally, fill materials associated with the previous development were encountered at the site. The fill encountered in the borings appeared relatively firm and free of debris. Even with the recommended site improvement and additional testing, some existing fill may remain in place in pavement areas. Construction on existing fill represents an inherent risk for the owner that compressible fill or unsuitable material within or buried by the fill will not be discovered. This risk of unforeseen conditions cannot be eliminated without completely removing the existing fill. However, these risks can be reduced by performing additional testing and evaluation during construction.

We understand the below ground level will include a concrete slab-on-grade. The varied depth to partially weathered rock can create stress concentration within the floor slab and lead to premature distress. To reduce the potential for stress concentrations to develop within the floor slab, and to increase the constructability of utilities, the floor slab design should include a crushed stone base course (NCDOT CABC) layer approximately 12 inches thick that extends to an approved subgrade.

Soils of the Triassic Basin are moisture sensitive and will become soft and unstable when wet.

Due to the potential for unstable subgrades during wet weather, it is preferable to perform earthwork operations during late summer and early fall when precipitation durations are shorter and drying times are quicker. Performing site preparation and earthwork at other times of the year increases the potential for having to perform remedial work on the subgrade soils.

We understand that heavy duty pavements will be constructed as part of the proposed project.

Prior to pavement construction the pavement subgrade should be proofrolled under observation by a representative of the geotechnical engineer to identify unsuitable areas. We expect that some localized areas may require additional overexcavation and replacement due to soft conditions.

A qualified geotechnical engineer should be retained during the construction phase of the project to observe earthwork and to perform necessary tests and observations during subgrade preparation; to monitor proof-rolling, placement and compaction of controlled compacted fills, foundation excavation operations, backfilling of excavations to the completed subgrade; and to observe prior to placing reinforcing steel in the footing excavations.

A more complete discussion of these points and additional information is included in the following sections.

4.2 Earthwork

demolition. As part of the demolition operation, the existing building foundations and slabs

Responsive ■ Resourceful ■ Reliable 6 should be removed. We understand the existing retaining wall and associated foundations will also be removed. Existing utilities that are to be abandoned should be removed or filled with grout. Utilities that are to remain in service should be accurately located horizontally and vertically to minimize conflict with new construction.

Existing topsoil and rootmat should be stripped and removed from construction areas. Stripping depths may vary and should be evaluated by a Terracon representative at the time of construction. Complete root systems of the existing trees should be removed.

The site should be proofrolled at accessible locations under observation of a representative of the geotechnical engineer to identify unsuitable materials. Proofrolling should be performed in areas to receive fill or at the subgrade elevation in cut areas. We anticipate proofrolling will be feasible within parking areas. Proofrolling in these areas should be performed as outlined in

Section 4.7 Heavy Duty Pavements. Deep excavations are planned within the footprint of the structure. The subgrade within the structure footprint should be evaluated as outlined in Section

4.3 Foundation Recommendations.

4.2.1 Excavations

Partially weathered rock (PWR) was encountered at elevations of 368 to 392 feet in the borings performed at the site. Excavation of PWR will be required in some portions of the proposed building footprint to reach the subgrade elevation for foundations and floor slabs and PWR may be encountered in utility or other excavations. Penetrating PWR will require additional effort such as ripping, jackhammering, or other means to advance excavations.

Free water was not encountered in the open boreholes at the site. However, ponded and perched water conditions can potentially develop on the project site when precipitation becomes trapped over the relatively impermeable soil and weathered rock. Perched or ponded water conditions that develop within the basement excavation will require dewatering. Dewatering measures may consist of installing a series of sump pumps.

We understand that the proposed construction limits for the project may extend to adjacent existing sidewalks, streets, and other structures. Excavation depths for the below grade portion of the proposed structure will likely not be possible without shoring or bracing. Alternatively, soil nail walls may be used to stabilize excavations as we understand that permanent soil nail walls are being considered for incorporation into the project to reduce earth pressures on basement walls. All temporary excavations that may be required during construction should comply with applicable local, state and federal safety regulations, including the current OSHA Excavation and Trench Safety Standards to provide stability and safe working conditions. The grading contractor, by his contract is usually responsible for excavation safety. The partially weathered rock of the Triassic basin can degrade and become unstable when exposed to moisture. A

Responsive ■ Resourceful ■ Reliable 7 qualified shoring contractor familiar with the geology of the Triassic Basin should be consulted to address these issues.

4.2.2 Acceptable Engineered Fill Materials

An approved subgrade will be suitable for placement of engineered fill. Engineered fill should meet the following material property requirements:

Fill Type USCS Classification Acceptable Location for Placement

Imported Low- to

Moderate-

Plasticity Soil with at least 15%

Fines

CL, ML, SC or SM As fill beneath new pavements, behind retaining walls, and in non-foundation bearing areas.

Sand / Gravel with less than 15% fines

(silt and clay)

GW/GP, SW/SP

NCDOT CABC (crushed aggregate base course) should be used beneath floor slabs, within pavement sections, and as a replacement material in foundation overexcavation areas.

NCDOT No. 57 (washed, crushed stone) may be used in conjunction with a geotextile fabric in water collection/drainage areas.

On-site soils with at least 15%

Fines

CL, CH, SC, SM

and

LL < 60 & PI < 30

As fill beneath new pavements, behind retaining walls, and in non-foundation bearing areas.

1. Controlled, compacted fill should consist of approved materials that are free of organic matter and debris. A sample of each material type should be submitted to the geotechnical engineer for evaluation.

2. “Clean” sand (less than 15% silt and clay) should not be used as general site fill in building and pavement areas to reduce risk of perched water developing in the surface fill as water infiltrating the surface zone becomes trapped above the less permeable sandy clay and clayey sand zone.

3. NCDOT No. 57 may also be used in conjunction with soil nail walls as described in Section 4.4

Lateral Earth Pressures

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4.2.3 Engineered Fill Compaction Requirements

Item Description

Fill Lift Thickness 9-inches or less in loose thickness (4” to 6” lifts when hand-operated equipment is used)

Compaction Requirements

Minimum of 98% of the materials standard Proctor maximum dry density (ASTM D698)

Moisture Content

Within the range of -3% to +3% of optimum moisture content as determined by the standard Proctor test at the time of placement and compaction

1. Engineered fill should be tested for moisture content and compaction during placement by a representative of the geotechnical engineer. If in-place density tests indicate the specified moisture or compaction limits have not been met, the area represented by the tests should be reworked and retested as required until the specified moisture and compaction requirements are achieved.

2. The compaction of backfill against retaining walls may be reduced to a minimum of 95% of the materials satandard Proctor maximum dry density to avoid overcompaction leading to excessive lateral earth pressures.

4.2.4 Grading and Drainage

Where possible, during construction, grades should be sloped to promote runoff away from the construction area. To the extent possible, the subgrade in the open basement excavation should be protected from moisture. Final grades should be sloped away from the structure on all sides to prevent ponding of water. Gutters / downspouts should not discharge directly adjacent to the building in landscaped areas. This can be accomplished through the use of splash-blocks, downspout extensions, and flexible pipes that are designed to day-light in such a manner that it gravity-drains collected water to a suitable outlet such as an approved storm sewer. Paved surfaces which adjoin the building should be sealed with caulking or other sealant to prevent moisture infiltration at the building envelope; maintenance should be performed as necessary to maintain the seal.

Recommendations regarding drainage for basement walls are discussed in Section 4.4 Lateral

Earth Pressures.

4.2.5 Construction Considerations

Terracon should be retained during the construction phase of the project to observe earthwork and to perform necessary tests and observations during subgrade preparation; to monitor proof-rolling, placement and compaction of controlled compacted fills, foundation excavation operations, backfilling of excavations to the completed subgrade; and to observe prior to placing reinforcing steel in the footing excavations. It is critical the each foundation excavation be observed by a representative of the geotechnical engineer in order for the foundation recommendations outlined in this report to be valid.

sshelton Oval

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Soils of the Triassic Basin are moisture sensitive and will become soft and unstable when wet.

Due to the potential for unstable subgrades during wet weather, it is preferable to perform earthwork operations during late summer and early fall when precipitation durations are shorter and drying times are quicker. Performing site preparation and earthwork at other times of the year increases the potential for having to perform remedial work on the subgrade soils.

4.3 Foundation Recommendations

We recommend support of the structure on shallow foundations bearing at an allowable bearing pressure of 3,000 pounds per square foot on weathered rock, fill consisting of crushed aggregate base course (NCDOT CABC) extending to weathered rock, or fill consisting of crushed aggregate base course (NCDOT CABC) extending to a depth of 5 feet below the design foundation bearing elevation. The recommended overexcavation and replacement with

CABC is intended to reduce the potential for development of premature distress in the structure due to differential settlement caused by varying stiffness of the subgrade materials at foundation bearing elevations. It is critical that the preparation of each foundation excavation be observed by a representative of the geotechnical engineer in order for the foundation recommendations outlined in this report to be valid. Recommendations for foundation design and construction are presented in the following sections.

4.3.1 Foundation Excavation Preparation

As previously outlined, excavations of approximately 5 to 15 feet below the existing surface will be required to reach the subgrade elevation for the below grade portion of the proposed structure. Foundation excavation should be performed under observation of a representative of the geotechnical engineer to confirm that suitable bearing conditions are encountered and to recommend necessary remedial measures.

A portion of the areas that require excavation are expected to encounter partially weathered rock (PWR) at foundation bearing elevations. Foundation excavations where partially weathered rock is encountered at bearing elevations will be suitable for support of spread footings. We anticipate that PWR will be encountered at foundation bearing elevations at locations in the vicinity of Boring Nos. B-2, B-3, and B-5. Where PWR is not encountered, foundations should be overexcavated to a depth recommended by the representative of the geotechnical engineer and replaced with NCDOT CABC. In general, overexcavation depths should extend to the top of

PWR or to a maximum depth of 5 feet below the planned foundation bearing elevation. We anticipate that a few feet of overexcavation in the vicinity of Boring No. B-1 will be required to reach the top of PWR. At other locations, where subsurface conditions are similar to those encountered in Boring No. B-4, overexcavation to depths of 5 feet below foundation bearing elevations will be required.

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When overexcavated areas do not extend to PWR, overexcavation for compacted structural fill placement below footings should extend laterally beyond all edges of the footings at least 8 inches per foot of overexcavation depth below footing base elevation. The overexcavation should then be backfilled up to the footing base elevation with compacted crushed aggregate base course (NCDOT CABC). The overexcavation and backfill procedures are described in the following figure.

Figure 1: Foundation overexcavation procedure

The base of all foundation excavations should be free of water and loose soil prior to placing concrete. Concrete should be placed soon after excavating to reduce bearing soil disturbance.

Should the soils at bearing level become excessively disturbed or saturated, the affected soil should be removed prior to placing concrete. Triassic basin PWR is susceptible to degradation with exposure to moisture.

Note: Excavations in sketches shown vertical for convenience. Excavations should be sloped as necessary for safety.

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4.3.2 Foundation Design Parameters

Design recommendations for a shallow foundation system are presented in the following table and paragraphs.

Description Value

Net allowable bearing pressure 3,000 psf

Minimum embedment below lowest adjacent finished grade

18 inches

Minimum width for continuous wall footings 16 inches

Minimum width for isolated column footings 24 inches

Approximate total settlement Less than 1 inch

Estimated differential settlement

Less than 1/2 inch between adjacent columns or over 40’ along wall footings

Coefficient of sliding friction 0.3

1. The recommended net allowable bearing pressure is the pressure in excess of the minimum surrounding overburden pressure at the footing base elevation.

2. For perimeter footings and footings beneath unheated areas.

3. The actual magnitude of settlement that will occur beneath the foundations would depend upon the variations within the subsurface soil profile, the structural loading conditions, and the quality of the foundation excavation.

The estimated total and differential settlements listed assume that the foundation related earthwork and the foundation design are completed in accordance with our recommendations.

4.3.3 Uplift Resistance

Uplift loads may be resisted by the dead weight of the structure and the weight of the soil above the footing in plan area. For compacted engineered fill, a unit weight of 120 pounds per cubic foot (pcf) may be used to calculate the weight of soil above the footing.

If additional uplift resistance is required, tiedown anchors could be utilized. Tiedown anchors are generally installed by drilling or coring into rock and embedding a portion of a threaded bar in grout or epoxy. Once the grout sets, the rod can be tensioned by threading the appropriate bearing plates and nuts. Anchors should be sleeved through the footings to prevent compressive loads from developing in the anchors.

Tiedown anchors are typically a design-build element. The grouted region of tiedown anchors are generally between 2 and 5 inches in diameter and the ultimate capacity of individual anchors typically range from 2 to 4 kips per foot of embedment in rock. An appropriate factor of safety should be applied to ultimate capacities and actual pullout strengths should be designed and verified by the design-build contractor. The tiedown anchor designer and installer should pay particular attention to the varied range of PWR and rock elevations encountered in the soil test borings. The installer should also provide detailed design calculations sealed by a professional engineer licensed in the State of North Carolina. The design calculations should

Responsive ■ Resourceful ■ Reliable 12 demonstrate that the system is capable of resisting the forces required by the structural design.

The specialty contractor should warrant their work. We recommend the design parameters for rock anchors be verified by installing test anchors and performing load testing.

4.4 Lateral Earth Pressures

We understand that lateral earth pressures may be used in the design of basement walls.

Foundation and retaining wall design should include the anticipated lateral earth pressures described in the following sections.

4.4.1 Basement Wall

One option in consideration for the basement wall of the structure includes a traditional reinforced concrete basement wall retaining soil backfill. The at-rest earth pressure conditions should be used for walls that are fixed or restrained from movement at the top such as basement walls. For active and passive earth pressures to develop, movement of the wall must occur. For engineered backfill, the following equivalent fluid pressures may be used for design:

At-rest (on-site or imported cohesive soils) ..................................................... 65 psf/ft

At-rest (imported granular with less than 15% fines) ....................................... 50 psf/ft

Active (on-site or imported cohesive soils) ...................................................... 45 psf/ft

Active (imported granular with less than 15% fines)........................................ 35 psf/ft

Passive (on-site or imported cohesive soils) ................................................. 300 psf/ft

Passive (imported granular with less than 15% fines) .................................. 375 psf/ft

Sliding friction between the subgrade and foundation base may be calculated using a coefficient of 0.3. The lateral earth pressures herein do not include any factor of safety and are not applicable for submerged soils/hydrostatic loading. For the granular values to be valid, the soil must extend away from the wall at an angle of 30 and 60 degrees from the horizontal for the active and passive case, respectively.

To reduce the potential for perched groundwater and surface water infiltration to impact foundation-bearing soils and to reduce hydrostatic loading on below grade walls, a subsurface drain system should be placed behind the below grade basement/retaining walls. The drain system should consist of free-draining, granular soils containing less than 5 percent fines (by weight) passing a No. 200 sieve placed adjacent to the wall. The free-draining, granular material should be encapsulated in a suitable filter fabric to prevent the intrusion of fines. Crushed, washed stone should be considered in areas where space is limited for compaction equipment such as between shoring and basement walls. A relatively impervious soil should be used in the

Responsive ■ Resourceful ■ Reliable 13 upper layer of backfill to reduce the potential for water infiltration. As an alternative, a prefabricated drainage structure such as geocomposite may be used as a substitute for the granular backfill adjacent to the wall. The drain system against walls should connect to a perimeter drain at the base of the foundation.

The perimeter drain should be constructed around the exterior perimeter of the foundation and sloped at a minimum of 1/8-inch per foot to a suitable outlet, such as an approved storm sewer or sump and pump system.

The perimeter drain should consist of a properly sized, perforated pipe that is embedded in free-draining gravel and placed in a trench at least 12 inches in width. Gravel should extend a minimum of 3 inches beneath the bottom of the pipe and at least 2 feet above the bottom of the foundation wall. The gravel should be covered with drainage fabric prior to placement of foundation backfill.

Fill against walls should be compacted to a minimum of 95% of the materials standard Proctor maximum dry density (ASTM D698). Compaction of each lift adjacent to walls should be accomplished with hand-operated tampers or other lightweight compactors in order to avoid overcompaction. Overcompaction may cause excessive lateral earth pressures, which could result in excessive stresses/loads and/or wall movement. The lateral earth pressures outlined above do not include a factor of safety or account hydrostatic pressure due to saturation of the backfill zone. The recommended drain system should be maintained to prevent build-up of water behind the walls.

4.4.2 Basement Wall in Conjunction with a Soil Nail Wall

We understand that two options currently under consideration for the basement walls include installing a permanent soil nail wall acting as a form for a cast-in-place concrete wall, and installing a permanent soil nail wall with backfill between the nail wall face and the independent basement wall.

Soil nail walls are typically installed under a design-build contract and many soil nail contractors can also install tiedown anchors. Soil nailing is a construction technique used to reinforce and strengthen existing ground by installing horizontal reinforcing elements to allow a vertical or near-vertical excavation. The use of soil nailing may be restricted if there are underground obstructions that would prevent the installation of the nails or legal restrictions such as property boundaries or easements.

The procedure for installing soil nails generally involves creating a series of shallow excavations

3 to 6 feet deep along the length of the wall. With a temporarily stable wall face, a series of tightly spaced holes are then drilled and filled with grout and a soil nail tendon. Drains, if

Responsive ■ Resourceful ■ Reliable 14 indicated in the wall design, are installed along the wall face prior to the installation of temporary shotcrete facing and securing of bearing plates and nuts. This process is repeated until the desired elevation is reached and a final facing is added for permanent walls. The following figure details the construction process for a soil nail wall:

Figure 2: Typical Nail Wall Construction Sequence (Adapted from FHWA Soil Nailing Field Inspectors Manual)

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The installer of the soil nail wall system should provide detailed design calculations sealed by a professional engineer licensed in the State of North Carolina. The design calculations should demonstrate that the soil nail system is capable of permanently retaining the soil mass with displacements less than what is required by the structural engineer. The specialty contractor should warrant their work as well as the total movement of the wall. We recommend the design parameters be verified by a full-scale pull out test.

Soil nail walls typically develop strength in tension from the steel reinforcing elements as the ground deforms laterally. Following the initial deformation of the soil nail wall and development of tension within the steel elements, the applied force on a basement wall cast against the soil nail wall should be very low. Actual forces should consider the anticipated creep of soil nail wall and should be verified by the wall designer.

Alternatively, a basement wall system consisting of permanent soil nail wall with backfill between the nailed face and independent basement wall may be utilized to allow for construction of perimeter basement wall foundations. By keeping the excavated face close to the building wall, the space between the excavated face and basement wall becomes more like a trench. Because of the arching effect in trench backfill, lateral stresses on the basement wall can be significantly reduced. For engineered backfill consisting of washed, crushed stone

(NCDOT No. 57), the following equivalent fluid pressure may be used for design:

At-rest............................................................................................................. 38 psf/ft

The applied at rest pressure does not include a factor of safety and does not account for hydrostatic loading. The backfill should be encapsulated in a suitable filter fabric to prevent the intrusion of fines. A relatively impervious soil should be used in the upper layer of backfill to reduce the potential for near-surface infiltration of water. The backfill should connect to a daylighted perimeter drain or sump and pump system to facilitate discharge to a suitable outlet, such as an approved storm sewer.

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4.5 Seismic Considerations

Code Used Seismic Site Classification

International Building Code (IBC) C

1. The International Building Code (IBC) site seismic classification is based on a site soil profile determination extending a depth of 100 feet. The scope of work authorized did not include a boring to a depth of 100 feet. The recommended seismic site classification is based the assumption that the structure will be supported on shallow foundations bearing on partially weathered rock (PWR) or equivalent materials and that the PWR encountered through max boring termination depths in the borings maintains or increases in density/stiffness through a depth of 100 feet. This is a reasonable assumption based on the geology of the area.

4.6 Floor Slabs

We understand the below ground level will include a concrete slab-on-grade. The varied depth to partially weathered rock at the design floor slab subgrade elevation can create stress concentration within the floor slab and lead to premature distress. To reduce the potential for stress concentrations to develop within the floor slab, and to increase the constructability of utilities, the floor slab design should include a crushed stone base course (NCDOT CABC) layer approximately 12 inches thick that extends to an approved subgrade. Excavation of PWR is expected to be required at some locations to reach the floor slab subgrade elevation.

Recommendations for design and construction of a concrete slab-on-grade are included in the following table and paragraphs:

Floor slab support / Stone base course 12 inches of crushed stone base course (NCDOT

CABC) over an approved subgrade.

Modulus of subgrade reaction (k) 150 pounds per square inch per inch (psi/in) for point loading conditions

We recommend floor subgrades be maintained in a relatively moist yet stable condition until floor slabs are constructed. If the subgrade should become excessively desiccated or wet prior to construction of floor slabs and pavements, the affected material should be removed or the materials scarified, moisture conditioned, and recompacted. Upon completion of grading operations in the building areas, care should be taken to maintain the recommended subgrade moisture content and density prior to construction of the building floor slabs. Saw-cut control joints should be placed in the slab to help control the location and extent of cracking. For additional recommendations, refer to the ACI Design Manual.

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The use of a vapor retarder should be considered beneath concrete slabs on grade that will be covered with wood, tile, carpet or other moisture sensitive or impervious coverings. The slab designer should refer to ACI 302 and/or ACI 360 for procedures and cautions regarding the use and placement of a vapor retarder.

4.7 Heavy Duty Pavements

We understand that heavy duty pavements will be constructed as part of the proposed project.

Prior to pavement construction the pavement subgrade should be proofrolled in areas to receive fill or at the subgrade elevation in cut areas. Proofrolling should be performed with a loaded, tandem-axle dump truck or similar rubber-tired construction equipment with a minimum gross weight of 20,000 lb. The proofrolling operations should be observed by a representative of the geotechnical engineer and should be performed after a suitable period of dry weather to avoid degrading an otherwise acceptable subgrade. Areas exhibiting excessive deflection or rutting, or areas where otherwise unsuitable material is encountered should be remediated as directed by the geotechnical engineer. We expect that some localized areas may require additional overexcavation and replacement due to soft conditions.

Pavement thickness design is dependent upon:

the anticipated traffic conditions during the life of the pavement;

subgrade and paving material characteristics; and climatic conditions of the region.

We have assumed that traffic loads at the site will be produced by lightly loaded cars, emergency vehicles, and delivery or sanitation trucks. Recommended pavement sections are listed in the table below.

Pavement Type Material Layer Thickness (inches)

Rigid

Portland Cement Concrete

(4,000 psi)

Crushed Aggregate Base Course

(NCDOT CABC Type 1 or Type 2)

Flexible

(Superpave)

Asphalt Surface

(NCDOT S-9.5B)

1.5

Asphalt Binder

(NCDOT I-19.0B)

3.5

Crushed Aggregate Base Course

(NCDOT CABC Type 1 or Type 2)

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The placement of a partial pavement thickness for use during construction is not suggested without a detailed pavement analysis incorporating construction traffic. In addition, we should be contacted to confirm the traffic assumptions outlined above. If the actual traffic varies from the assumptions outlined above, modification of the pavement section thickness will be required.

Recommendations for pavement construction presented depend upon compliance with recommended material specifications. To assess compliance, observation and testing should be performed under the direction of the geotechnical engineer.

Asphalt concrete aggregates and base course materials should conform to the North Carolina

Department of Transportation (NCDOT) "Standard Specifications for Roads and Structures.”

Concrete pavement should be air-entrained and have a minimum compressive strength of 4,000 psi after 28 days of laboratory curing per ASTM C-31.

The performance of all pavements can be enhanced by minimizing excess moisture which can reach the subgrade soils. The following recommendations should be considered a minimum:

site grading at a minimum 2 percent grade away from the pavements;

subgrade and pavement surface with a minimum 1/4 inch per foot (2%) slope to promote proper surface drainage; and installation of joint sealant to seal cracks immediately

Preventative maintenance should be planned and provided for through an ongoing pavement management program to enhance future pavement performance. Preventative maintenance activities are intended to slow the rate of pavement deterioration and to preserve the pavement investment. Preventative maintenance, which consists of both localized maintenance (e.g. crack and joint sealing and patching) and global maintenance (e.g. surface sealing), is usually the first priority when implementing a planned pavement maintenance program and provides the highest return on investment for pavements.

5.0 GENERAL COMMENTS

Terracon should be retained to review the final design plans and specifications so comments can be made regarding interpretation and implementation of our geotechnical recommendations in the design and specifications. Terracon also should be retained to provide observation and testing services during grading, excavation, foundation construction and other earth-related construction phases of the project.

The analysis and recommendations presented in this report are based upon the data obtained from the borings performed at the indicated locations and from other information discussed in

Responsive ■ Resourceful ■ Reliable 19 this report.